U.S. patent number 7,221,814 [Application Number 10/521,335] was granted by the patent office on 2007-05-22 for optical waveguide based surface profiling apparatus.
This patent grant is currently assigned to Aston University. Invention is credited to Thomas David Paul Allsop, Ian Bennion, Timothy Earthrowl-Gould.
United States Patent |
7,221,814 |
Allsop , et al. |
May 22, 2007 |
Optical waveguide based surface profiling apparatus
Abstract
Surface profiling apparatus (10) according to one embodiment
comprises three long period gratings (LPGs) (12, 14, 16) fabricated
in progressive three layered (PTL) fibre (18) and embedded within a
deformable carrier member (40) comprising a skeleton (42) provided
between two sheets of flexible rubber skin (44, 46). The LPGs (12,
14, 16) are illuminated by three wavelength modulated, narrow
bandwidth optical signals, each having a different wavelength and
modulation frequency. A photodetector (26) connected to three
lock-in amplifiers (28, 30, 32) measures the amplitudes of the
first and second harmonic frequency components of the photodetector
output signal corresponding to each LPG (12, 14, 16). Similar
surface profiling apparatus (10) forms the basis for respiratory
function monitoring apparatus (100) in which five LPGs are provided
within each of four PTL fibres (104, 106, 108, 110) and embedded in
four carrier members (40a d) attached to a garment (114) to be worn
by a subject.
Inventors: |
Allsop; Thomas David Paul
(Birmingham, GB), Earthrowl-Gould; Timothy
(Birmingham, GB), Bennion; Ian (Birmingham,
GB) |
Assignee: |
Aston University (Birmingham,
GB)
|
Family
ID: |
29797312 |
Appl.
No.: |
10/521,335 |
Filed: |
July 23, 2003 |
PCT
Filed: |
July 23, 2003 |
PCT No.: |
PCT/GB03/03256 |
371(c)(1),(2),(4) Date: |
January 10, 2005 |
PCT
Pub. No.: |
WO2004/008963 |
PCT
Pub. Date: |
January 29, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050244094 A1 |
Nov 3, 2005 |
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Foreign Application Priority Data
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Jul 23, 2002 [EP] |
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02255134 |
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Current U.S.
Class: |
385/13 |
Current CPC
Class: |
A61B
5/1077 (20130101); A61B 5/1135 (20130101); G02B
6/022 (20130101); G02B 6/02095 (20130101); G02B
6/29319 (20130101); G02B 6/29322 (20130101); G02B
6/29356 (20130101) |
Current International
Class: |
G02B
6/00 (20060101) |
Field of
Search: |
;356/73.1,345,352
;250/227.14,227.23 ;385/4,14,147 ;705/2 ;600/300 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1372006 |
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Dec 2003 |
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EP |
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WO 86/01303 |
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Feb 1986 |
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WO |
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WO 93/22624 |
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Nov 1993 |
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WO |
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WO 93/22624 |
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Nov 1993 |
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WO |
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WO 00/70307 |
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Nov 2000 |
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WO |
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WO 00/70307 |
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Nov 2000 |
|
WO |
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Other References
A B. Lobo Ribeiro et al., "General error function of
synthetic-heterodyne signal processing in interferometric
fibre-optic sensors", International Journal of Optoetectronics,
1995, vol. 10, No. 3, XP-000587864, pp. 205-209. cited by other
.
S. K. Yao et al., "Microbending loss in a single-mode fiber in the
pure-bend loss regime", Applied Optics, vol. 21, No. 17, Sep. 1,
1982, XP-001093774, pp. 3059-3060. cited by other.
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Primary Examiner: Connelly-Cushwa; Michelle
Assistant Examiner: Chu; Chris H.
Attorney, Agent or Firm: Buckley, Maschoff & Talwalkar
LLC
Claims
The inventon claimed is:
1. A surface profiling apparatus (10, 60, 80, 102, 120) comprising:
an optical waveguide including a plurality of sensor sections, each
sensor section comprising a respective optical waveguide grating
curvature sensing device, each optical waveguide grating curvature
sensing device comprising at least one long period grating (12, 14,
16); and optical interrogation means operable to interrogate the
optical waveguide grating curvature sensing devices to determine
the curvature experienced by each device, the optical interrogation
means comprising: an optical source optically coupled to one,
input, end (18a) of the respective optical waveguide and being
operable to generate a narrow spectral bandwidth optical signal at
a wavelength within the spectral range of an optical waveguide
grating curvature sensing device to be interrogated, the optical
signal being wavelength-modulated at a modulation frequency; and
optical detection means optically coupled to the other, output, end
(18b) of the optical waveguide and being operable to measure the
amplitude of a detected optical signal at least one harmonic of the
modulation frequency in order to detect changes in the spectral
transmission profile of the optical waveguide grating curvature
sensing device being interrogated and to thereby determine the
curvature experienced by the optical waveguide grating curvature
sensing device; whereby the sensor sections are couplable to a
surface to be profiled, and a profile of said surface is
constructed from the curvatures sensed by the optical waveguide
grating curvature sensing devices.
2. The apparatus of claim 1, wherein the optical waveguide is an
optical fibre (18) such as a silica-glass optical fibre or a
polymer optical fibre, the optical fibre (18) comprising a core, an
inner cladding layer surrounding the core, and at least a first
outer cladding layer surrounding the inner cladding layer, the
refractive index of the inner cladding layer being less than the
refractive index of the core, and the refractive index of the first
outer cladding layer being less than the refractive index of the
inner cladding layer.
3. The apparatus of claim 2, wherein the optical fibre (18) further
comprises a second outer cladding layer surrounding the first outer
cladding layer in order to isolate light propagating within a
cladding mode of the inner cladding layer from a medium surrounding
the second outer cladding layer, the refractive index of the second
outer cladding layer being less than the refractive index of the
first outer cladding layer.
4. The apparatus of claim 1, wherein the at least one long period
grating (12, 14, 16) comprises two long period gratings arranged to
together define an in-line Mach-Zehnder interferometer.
5. The apparatus of claim 1, further comprising coupling means for
coupling the sensor sections to the surface to be profiled, the
coupling means comprising a carrier member (40), and the sensor
sections of the optical waveguide being fixed to or embedded within
the carrier member (40).
6. The apparatus of claim 5, wherein the coupling means comprises a
plurality of carrier members (40) mounted on a support structure,
one or more sensor sections being fixed to or embedded within each
carrier member (40).
7. The apparatus of claim 6, wherein each carrier member (40) is
deforniable and comprises a flexible skin fixed to a partially
rigid, expandable skeleton structure.
8. The apparatus of claim 5, wherein the carrier member (40) is
deformable and comprises a flexible skin fixed to a partially
rigid, expandable skeleton structure.
9. The apparatus of claim 5, further comprising a respiratory
function monitoring apparatus (100) operable to utilize the surface
profiling apparatus (10,60, 80, 102, 120) for use on the rib cage
or torso during respiratory movement.
10. The apparatus of claim 1, wherein the optical interrogation
means is at least one of a derivative spectroscopy and a synthetic
heterodyne based optical interrogation means operable to detect
changes in the spectral profile of an optical waveguide grating
curvature sensing device.
11. The apparatus of claim 10, wherein the optical detection means
comprises: a photodetector (26) optically coupled to the output end
(18b) of the or each optical waveguide and a plurality,
corresponding to the number of optical waveguide grating curvature
sensing devices provided within the respective waveguide, of
lock-in amplifiers (28, 30, 32) or synchronous detectors each
operable to measure the amplitude of a detected optical signal at
the modulation frequency associated with a particular optical
waveguide grating curvature sensing device and a harmonic of the
modulation frequency; and data processing means connected to the
photodetector (26), operable to calculate the ratio of the
amplitudes and the arc tangent of the ratio of the amplitudes, to
which the curvature experienced by the optical waveguide grating
curvature sensing device under interrogation is linearly related.
Description
This application is a U.S. National Stage filing under 35 U.S.C.
.sctn.371 and 35 U.S.C .sctn.119, based on and claiming priority to
PCT/GB03/03256 for "OPTICAL WAVEGUIDE BASED SURFACE PROFILING
APPARATUS".
The invention relates to surface profiling apparatus and to
respiratory function monitoring apparatus incorporating the surface
profiling apparatus.
There are many situations in which there is a requirement to
measure or monitor the shape or profile of a surface. An important
example is the monitoring of respiratory function through
non-invasive measurement of thoracoabdominal surface movement.
Current approaches to carrying out volumetric measurement of
respiratory function include inductance plethysmography in which a
continuous, low-voltage electrical signal is passed through two
coils of wire placed around a subject's rib cage and abdomen
respectively. In this approach the changes in chest and abdomen
volumes are taken to be equivalent to the changes in metric of the
chest and abdomen compartments respectively. However, accurate
calibration of this type of device is difficult and it gives a
limited picture of the movements involved. Alternative approaches
using cameras to record the movement of markers or grids of light
on the chest have also been employed, but these are extremely
complex techniques and they do not allow free ambulatory movement
of the subject.
Various fibre-optic based devices have been developed which measure
respiration according to the two degree of freedom system
associated with inductance plethysmography. These devices perform
measurements of the perimeter of the chest and abdomen, or depth of
breathing, using optical fibre and fibre Bragg gratings as strain
or displacement transducers. This technique ignores distortions
which occur in the rib cage with increasing volumes of ventilation
and relies on additional parameters which compensate for
geometrical factors, cumulatively lumping them as part of
coefficients in most cases as part of a calibration procedure.
Whilst these methods can typically provide accurate results (.+-.5%
error in tidal volume) for a single set of stationary quiet
breathing conditions, accuracy is degraded with changes in posture
as well as breathing pattern and magnitude. In validation studies
of devices using these techniques, errors of as much as .+-.30% in
respired tidal volume were not uncommon in a significant number of
samples (up to 20% of the sample population). Such devices must
also be re-calibrated frequently if the results produced are to be
regarded as more than qualitative. As such they generally find
application as monitors for recumbent patients, for example
monitoring of sleep and postoperative apnoeas.
According to a first aspect of the invention there is provided
surface profiling apparatus comprising: a first optical waveguide
including a plurality of sensor sections in which a plurality of
optical waveguide grating curvature sensing devices are
respectively provided; and optical interrogation means operable to
interrogate the optical waveguide grating curvature sensing
devices, to determine the curvature experienced by each device,
whereby a profile of a surface to which the sensor sections of the
first optical waveguide are coupled may be constructed from the
curvatures sensed by the optical waveguide grating curvature
sensing devices.
The optical waveguide is preferably an optical fibre, which may be
a silica-glass optical fibre or a polymer optical fibre.
The surface may be an exterior surface or may be an internal
surface within a structure.
The first optical fibre preferably comprises a core, an inner
cladding layer surrounding the core, and at least a first outer
cladding layer surrounding the inner cladding layer, the refractive
index of the inner cladding layer being less than the refractive
index of the core, and the refractive index of the first outer
cladding layer being less than the refractive index of the inner
cladding layer.
Desirably, the first outer cladding layer is thick enough to
isolate light propagating within a cladding mode of the inner
cladding layer from a medium surrounding the first outer cladding
layer. The first optical fibre may be progressive three layered
optical fibre or matched index optical fibre.
The first optical fibre may further comprise a second outer
cladding layer surrounding the first outer cladding layer in order
to isolate light propagating within a cladding mode of the inner
cladding layer from a medium surrounding the outermost cladding
layer, the refractive index of the second outer cladding layer
being less than the refractive index of the first outer cladding
layer. The first optical fibre may comprise a plurality of outer
cladding layers, each outer cladding layer surrounding a preceding
outer cladding layer and having a lower refractive index than the
preceding outer cladding layer.
The refractive index profile of the fibre core may be radially
asymmetric. Alternatively or additionally the refractive index
profile of one or more cladding layers of the fibre may be radially
asymmetric.
The surface profiling apparatus may further comprise an optical
waveguide strain sensor, and most preferably further comprises a
plurality of optical waveguide strain sensors. One or more optical
waveguide strain sensors may be provided within the first optical
waveguide. A strain sensor is preferably provided between a or each
pair of optical waveguide grating curvature sensing devices.
One or more optical waveguide strain sensors may alternatively or
additionally be provided within a second optical waveguide, which
may be a second optical fibre. A strain sensor may be provided
between a or each pair of optical waveguide grating curvature
sensing devices. Alternatively or additionally, a strain sensor may
be provided generally adjacent to, and generally parallel with, a
or each optical waveguide grating curvature sensing device.
The first optical fibre may alternatively comprise an asymmetric
optical fibre, which may have a radially asymmetric core or a
radially asymmetric cladding layer. The first optical fibre is
preferably D-shaped optical fibre.
The surface profiling apparatus may further comprise an optical
waveguide strain sensor provided within a second optical waveguide.
The surface profiling apparatus preferably comprises a plurality of
optical waveguide strain sensors, which may be provided within a
plurality of second optical waveguides. The or each second optical
waveguide may comprise a second optical fibre. A strain sensor may
be provided between a or each pair of optical waveguide grating
curvature sensing devices. Alternatively or additionally, a strain
sensor may be provided generally adjacent to, and generally
parallel with, a or each optical waveguide grating curvature
sensing device.
The or each optical waveguide strain sensor is preferably an
optical waveguide grating strain sensor, and is most preferably a
Bragg grating.
A plurality of first optical waveguides including a plurality of
sensor sections may be provided.
The optical waveguide grating curvature sensing devices preferably
comprise optical waveguide grating devices. An optical waveguide
grating device may comprise a long period grating. The long period
grating may be radially asymmetric. The long period grating may
include one or more phase-shifts within its periodic refractive
index variation. Alternatively or additionally the period of the
refractive index variation of one or more parts of the long period
grating may be chirped.
An optical waveguide grating device may alternatively comprise two
long period gratings arranged to together define an in-line
Mach-Zehnder interferometer. An optical waveguide grating device
may further alternatively comprise an optical waveguide Bragg
grating. The Bragg grating may be a chirped Bragg grating.
Alternatively, or additionally, the amplitude of the periodic
refractive index variation of the Bragg grating may be tapered
and/or apodised. An optical waveguide grating device may further
alternatively comprise two optical waveguide Bragg gratings
arranged to together define a Fabry-Perot etalon.
The surface profiling apparatus may further comprise coupling means
for coupling the sensor sections of the first optical waveguide to
the surface to be profiled. The surface profiling apparatus may
further comprise additional coupling means for coupling the or each
optical waveguide strain sensor to the surface to be profiled. A
coupling means preferably comprises a carrier member, one or more
sensor sections of the optical waveguide or one or more optical
waveguide strain sensors being fixed to or embedded within a
carrier member. The coupling means may alternatively comprise a
plurality of carrier members mounted on a support structure, one or
more optical waveguide sensor sections or one or more optical
waveguide strain sensors being fixed to or embedded within each
carrier member. The or each carrier member is preferably deformable
and most preferably comprises a flexible skin fixed to a partially
rigid, expandable skeleton structure. The carrier member or support
structure is preferably of a corresponding size and shape to the
surface to be profiled, such that a close fit is provided between
the or each carrier member and the surface.
The optical interrogation means is preferably a derivative
spectroscopy or synthetic heterodyne based optical interrogation
means operable to detect changes in the spectral profile of an
optical waveguide grating curvature sensing device. The optical
interrogation means preferably comprises an optical source operable
to generate a wavelength modulated optical signal at a wavelength
within the spectral range of an optical waveguide grating curvature
sensing device to be interrogated, the optical source being
optically coupled to one, input, end of the respective optical
waveguide, and optical detection means optically coupled to the
other, output, end of the optical waveguide and being operable to
detect changes in the spectral transmission profile of the optical
waveguide grating curvature sensing device being interrogated and
to thereby determine the curvature experienced by the grating
curvature sensing device.
The optical signal preferably has a narrow spectral bandwidth
compared with the spectral bandwidth of the optical waveguide
grating curvature sensing device to be interrogated.
The optical interrogation means is preferably further operable to
interrogate the or each optical waveguide strain sensor, to
determine the strain experienced by each strain sensor. The optical
interrogation means may further comprise a second optical detection
means optically coupled to the input end of the optical waveguide
and being operable to detect changes in the spectral reflection
profile of an optical waveguide strain sensor being interrogated,
to thereby determine the strain experienced by the strain
sensor.
The optical source may comprise a plurality of wavelength modulated
lasers, the wavelength of each laser output optical signal lying
within the spectral range of its respective optical waveguide
grating curvature sensing device or optical waveguide strain
sensor. One or each of the wavelength modulated lasers may be
distributed feedback lasers, the injection current provided to the
laser from its drive unit being modulated at a desired frequency to
thereby produce a wavelength modulation on the optical output
signal generated by the laser. One or each of the wavelength
modulated lasers may alternatively comprise fibre lasers having a
fibre Bragg grating for one or both of the laser mirrors, the or
each fibre Bragg grating being coupled to tuning means operable to
vary the resonant wavelength of the or each fibre Bragg grating at
a desired modulation frequency, thereby apply a corresponding
wavelength modulation to the laser output signal.
The optical source may alternatively or additionally comprise a
plurality of fibre Bragg gratings, each grating having a different
resonant wavelength lying within the spectral profile of a
respective optical waveguide grating curvature sensing device or
optical waveguide strain sensor, and being coupled to tuning means
operable to vary its resonant wavelength at a desired modulation
frequency, and a broad bandwidth optical source for illuminating
the gratings, the light reflected by each grating thereby forming a
wavelength modulated narrow bandwidth optical signal. The broad
bandwidth optical source may be a superluminescent light emitting
diode or an edge-emitting light emitting diode.
Preferably, a different modulation frequency is used for each
optical waveguide grating curvature sensing device or optical
waveguide strain sensor provided in a single optical waveguide.
The optical source may further alternatively comprise a wavelength
tunable optical source, operable to generate a narrow bandwidth
optical signal, and wavelength modulation apparatus operable to
apply a wavelength modulation at a desired modulation frequency to
the generated optical signal. The wavelength tunable optical source
may be a distributed feedback laser or a Fabry-Perot etalon based
laser.
The optical detection means preferably comprises: a first
photodetector optically coupled to the output end of the or each
optical waveguide; and a plurality, corresponding to the number of
optical waveguide grating curvature sensing devices provided within
the respective waveguide, of lock-in amplifiers or synchronous
detectors each operable to measure the amplitude of a detected
optical signal at the modulation frequency associated with a
particular optical waveguide grating curvature sensing device and a
harmonic of the modulation frequency, most preferably the second
harmonic.
The optical interrogation means may additionally comprise second
optical detection means, in the form of a second photodetector,
optically coupled to the input end of the optical waveguide.
The optical detection means preferably further comprises data
processing means connected to the or each first photodetector,
operable to calculate the ratio of the amplitudes. The data
processing means is desirably further operable to calculate the are
tangent of the ratio of the amplitudes, to which the curvature
experienced by an optical waveguide grating curvature sensing
device under interrogation is linearly related.
The optical interrogation means may alternatively comprise: a
broadband optical source operable to generate a broad bandwidth
optical signal having a spectral bandwidth encompassing the
spectral profile of an optical waveguide grating curvature sensing
device or optical waveguide strain sensor to be interrogated, the
optical source being optically coupled to one, input, end of the
respective optical waveguide; and optical detection means optically
coupled to the other, output, end of the optical waveguide.
The optical interrogation means may additionally comprise second
optical detection means optically coupled to the input end of the
optical waveguide. The optical detection means preferably comprises
an optical spectrum analyser operable to record the spectral
profile of the optical waveguide grating curvature sensing device
or optical waveguide strain sensor under interrogation and data
processing means, such as a microprocessor or personal computer,
operable to match the recorded spectral profile with one of a
plurality of pre-recorded spectral profiles, to thereby determine
the curvature or strain experienced by the grating device under
interrogation.
The optical spectrum analyser and the data processing means are
preferably portable.
The data processing means is preferably further operable to
generate a two-dimensional or three-dimensional wire-frame profile
of the surface being interrogated from the curvature values. The
data processing means is preferably additionally operable to
generate a two-dimensional or three-dimensional wire-frame profile
of the surface being interrogated from the curvature values and the
strain values.
According to a second aspect of the invention there is provided
respiratory function monitoring apparatus comprising surface
profiling apparatus according to the first aspect of the
invention.
Preferably, the support structure of the coupling means comprises a
garment of a size and shape suitable to closely fit across at least
part of the thoracoabdominal surface of a subject whose respiratory
function is to be monitored.
The data processing means is preferably operable to generate a 2-
or 3-dimensional wire-frame image of the thoracoabdominal surface
of a subject wearing the respiratory function monitoring apparatus,
and is most preferably operable to repeatedly generate the image in
real time, to thereby generate a changing, updating image of the
thoracoabdominal surface.
Embodiments of the invention will now be described in detail, by
way of example only, with reference to the accompanying drawings,
in which:
FIG. 1 is a schematic representation of surface profiling apparatus
according to a first embodiment of the invention;
FIG. 2 illustrates the effect on the transmission spectrum of a
long period grating (period 480 .mu.m) of the application of
various curvatures (C m.sup.-1) to the grating;
FIG. 3 shows the attenuation profile of an optical waveguide
grating device in the form of a long period grating for three
different applied curvatures (C.sub.1, C.sub.2 and C.sub.3)
together wavelength modulation range of a wavelength modulated
narrow bandwidth optical signal;
FIG. 4 shows a parametric plot of the first and second harmonics of
the wavelength modulation frequency for various curvatures applied
to a long period grating;
FIG. 5 shows the arctan of the ratio (R) of the amplitudes of the
first and second harmonics of the wavelength modulation frequency
as a function of radius of curvature (C) for a long period grating
of period 240 .mu.m;
FIG. 6 shows the arctan of the ratio (R) of the amplitudes of the
first and second harmonics of the wavelength modulation frequency
as a function of radius of curvature (C) for a long period grating
of period 480 .mu.m;
FIG. 7 is a diagrammatic partially exploded plan view of a carrier
member suitable for use with the apparatus of FIG. 1;
FIG. 8 is a diagrammatic sectional view along the line A--A of FIG.
7;
FIG. 9 shows the central wavelength of the attenuation band
(4.sup.th mode) of the third LPG 16 (period 350 .mu.m) as a
function of radius of curvature;
FIG. 10 is a schematic representation of surface profiling
apparatus according to a second embodiment of the invention;
FIG. 11 is a schematic representation of surface profiling
apparatus according to a third embodiment of the invention;
FIG. 12 is a schematic representation of surface profiling
apparatus according to a fourth embodiment of the invention;
FIG. 13 is a schematic representation of surface profiling
apparatus according to a fifth embodiment of the invention;
FIG. 14 is a schematic representation of surface profiling
apparatus according to a sixth embodiment of the invention;
FIG. 15 is a schematic representation of surface profiling
apparatus according to a seventh embodiment of the invention;
FIG. 16 is a diagrammatic representation of respiratory function
monitoring apparatus according to a eighth embodiment of the
invention;
FIG. 17 shows plots of percentage surface area error (E) as a
function of number of monitoring locations for reconstructed CT
scan chest profile data: (A) trapezoidal approximation; and (B)
4-point cubic spline interpolation;
FIG. 18 is a diagrammatic representation of a torso showing the
location of the carrier member on the upper chest (X) and the lower
chest (Y);
FIG. 19 shows the change in the central wavelength (.DELTA..lamda.)
of the LPG's attenuation band as a function of the change in the
circumference (.DELTA.c) of the torso, for the carrier member
located on the lower chest (positions 2, 3 & 5); and
FIG. 20 shows the change in the central wavelength (.DELTA..lamda.)
of the LPG's attenuation band as a function of the change in the
circumference (.DELTA.c) of the torso, for the carrier member
located on the upper chest (positions 1 & 4).
Referring to FIGS. 1 to 9, a first embodiment of the invention
provides surface profiling apparatus 10 utilising optical waveguide
grating devices in the form of long period gratings (LPGs) 12, 14,
16 provided within respective sensing sections of single mode
progressive three layered (PTL) optical fibre 18. Only three LPGs
provided within a single carrier member are shown here for clarity
but it will be appreciated that a larger number of LPGs may be used
and may be provided within one or more carrier members.
LPGs consist of a periodic refractive index variation produced
within the core of an optical fibre. The refractive index variation
is induced within the fibre as a result of exposure of the fibre to
ultra-violet radiation. The period of the refractive index
variation is typically between 100 .mu.m and 600 .mu.m, and is much
greater than the guided wavelength. An LPG acts to couple light
incident on it from the fibre core into the fibre cladding, thereby
producing attenuation bands within the transmission spectrum of the
optical fibre. Light is coupled from the core into the cladding
with a spectral selectivity that is closely determined by the
periodicity of the refractive index variation.
LPGs are sensitive to strain (.epsilon.), temperature (T) and the
refractive index (n.sub.S) of the surrounding medium. The
sensitivity of an LPG to these parameters can manifest itself in
two different ways: the central wavelength of the attenuation band
can shift in wavelength; and a change in the spectral transmission
profile of the attenuation band can occur. Of particular interest
here is the sensitivity of LPGs to bending, which induces both a
wavelength shift and a change in the spectral profile of the
attenuation band.
The wavelength shift of the attenuation band arises as a result of
the phase match condition of an LPG, which determines the spectral
position of the attenuation band, and is given by
[n(co).sub.eff(.lamda.,T,.lamda.)-n.sup..nu.(cl).sub.eff(.lamda.,T,n.sub.-
S,.epsilon.)].LAMBDA.(T,
.epsilon.)=.DELTA.n.sub.eff.LAMBDA.(T,.epsilon.)=.lamda. (1) where
.LAMBDA. is the period of the grating, n(co).sub.eff is the
effective refractive index of the core mode and
n.sup..nu.(cl).sub.eff is the effective refractive index of the
.nu..sup.th radial cladding mode, both indices also being dependent
on the refractive indices of the core and cladding, and on
wavelength .lamda..
The magnitude of the wavelength shift induced by an applied strain,
or a change in temperature or the refractive index of the
surrounding medium, is dependent on the difference between the
effective refractive indices of the core and the .nu..sup.th radial
cladding mode, and on the difference between the group effective
refractive indices of the core and .nu..sup.th radial cladding
modes. The wavelength sensitivity of LPGs to bending arises from
their sensitivity to strain. Bending an optical fibre induces
strain and compression in the fibre, which in turn changes the
group effective refractive indices of the core and the .nu..sup.th
radial cladding mode as well as n(co).sub.eff and
n.sup..nu.(cl).sub.eff.
In the embodiment shown in FIG. 1, the first LPG 12 has a period of
240 .mu.m, a length of 8 cm and a strength of .about.14 dB, the
second LPG 14 has a period of 480 .mu.m, a length of 10 cm and a
strength of a .about.10 dB, and the third LPG 16 has a period of
350 .mu.m, a length of 10 cm and a strength of .about.10 dB. The
LPGs 12, 14, 16 were fabricated using the point-to-point
fabrication technique which will be well known to the skilled
person and so will not be described in detail here. The PTL fibre
used was not specifically designed to be photosensitive and so its
photosensitivity was increased by hydrogenation at a pressure of
120 Bar for a period of 2 weeks at room temperature.
The periods of the LPGs 12, 14, 16 were chosen so that the
associated cladding modes of the attenuation bands were from modes
n.sub.cl(1,1) to n.sub.cl(1,10), which are known to be insensitive
to the refractive index n.sub.s of the surrounding medium.
The first LPG 12 produces an attenuation band having a central
wavelength of .about.1536 nm, associated with its 9.sup.th cladding
mode, the second LPG 14 produces an attenuation band having a
central wavelength of .about.1522 nm, associated with its 5.sup.th
cladding mode, and the third LPG 16 produces an attenuation band
having a central wavelength of .about.1520 nm, associated with its
4th cladding mode.
FIG. 2 shows how the optical transmission spectrum (T) of the
second LPG 14 changes as the radius of curvature applied to the LPG
14 is increased from 0 to 3.356 m.sup.-1.
In this example, the optical interrogation means takes the form of
three distributed feedback (DFB) lasers 20, 22, 24 optically
coupled to the input end 18a of the PTL fibre 18. The DFB lasers
20, 22, 24 are thermally stabilised and optical fibre pigtailed,
and are operable to generate wavelength modulated, narrow bandwidth
(i.e. narrow with respect to the spectral bandwidths of the LPGs
12, 14, 16 to be interrogated) optical signals. Both the wavelength
of the optical signal generated by each DBF laser 20, 22, 24 and
the frequency of the applied wavelength modulation are different
for each DFB laser 20, 22, 24, and thus each respective LPG 12, 14,
16. The wavelength of each optical signal is selected to be close
to the resonant wavelength of the respective LPG 12, 14, 16. The
output fibre pigtails 20a, 22a, 24a of the DFB lasers 20, 22, 24
are optically coupled to the PTL fibre 18 via a 3.times.1 optical
fibre multiplexer 36. A photodetector 26 is optically coupled to
the output end 18b of the PTL fibre 18. The electrical output of
the photodetector 26 is connected to three lock-in amplifiers 28,
30, 32. The different wavelength modulation frequencies are used to
identify the LPG 12, 14, 16 which each part of the output signal
generated by the photodetector 26 relates to, as will described in
more detail below.
The first DFB laser 20 generates a narrow bandwidth optical signal
having a wavelength .lamda..sub.1 of .about.1532 nm. An electrical
sinusoidal modulation signal of a frequency .omega..sub.1 of 5 kHz,
generated by a signal generator 34, is applied to the laser
injection current to thereby apply a sinusoidal wavelength
modulation, having a first harmonic frequency of 5 kHz and an
amplitude of 0.06 nm, to the optical signal. Modulating the
wavelength at a particular modulation frequency .omega..sub.1
generates wavelength modulations on the optical signal at a series
of harmonics of the modulation frequency i.e. .omega..sub.1,
2.omega..sub.1 etc. The second DFB laser 22 generates a narrow
bandwidth optical signal having a wavelength .lamda..sub.2 of
.about.1517 nm. An electrical sinusoidal modulation signal of a
frequency .omega..sub.2 of 3.7 kHz, generated by the signal
generator 34, is applied to the laser injection current to thereby
produce a sinusoidal wavelength modulation having a first harmonic
frequency of 3.7 kHz and an amplitude of 0.06 nm. The third DFB
laser 24 generates a narrow bandwidth optical signal having a
wavelength .lamda..sub.3 of .about.1415 nm. An electrical
sinusoidal modulation signal of a frequency .omega..sub.3 of 2.3
kHz, generated by the signal generator 34, is applied to the laser
injection current to thereby produce a sinusoidal wavelength
modulation having a first harmonic frequency of 2.3 kHz and an
amplitude of 0.06 nm. The drive current is set to operate in the
saturation regimes of the DFB lasers 20, 22, 24 where the current
induced amplitude modulation is minimised.
The series of wavelength modulation frequency harmonics present on
each optical signal give rise to corresponding frequency components
in the electrical output signal from the photodetector 26. The
in-phase component of the n.sup.th harmonic frequency component of
the photodetector output signal is proportional to the n.sup.th
derivative of the spectral profile under interrogation. That is to
say, the amplitudes of the first and second harmonic frequency
components (.omega..sub.1 and 2.omega..sub.1, .omega..sub.2 and
2.omega..sub.2, and .omega..sub.3 and 2.omega..sub.3) of the
photodetector output signal are proportional to the first and
second derivatives of the spectral transmission profiles of the
respective LPGs 12, 14, 16.
FIG. 3 shows the attenuation profiles (percentage of transmission
power (T) as a function of wavelength (.lamda.)) of an LPG for
three different curvatures (C.sub.1, C.sub.2, and C.sub.3),
together with the wavelength modulation 38 at frequency .omega. of
a DFB laser, which may be given by:
.lamda..sub.DFB=.lamda..sub.0+.delta..lamda. sin(.omega.t)
It can be seen in FIG. 3 that the amplitude A.sub.1, A.sub.2, and
A.sub.3 of the photodetector output signal at frequency co varies
with the amount of curvature applied to the LPG.
In addition to the signals at modulation frequencies .omega..sub.1,
.omega..sub.2 and .omega..sub.3, the signal generator 34 also
generates sinusoidal electrical signals at the second harmonics of
the modulation frequencies i.e. at 2.omega..sub.1, 2.omega..sub.2
and 2.omega..sub.3. The electrical signals generated by the signal
generator 34 at frequencies .omega..sub.1 and 2.omega..sub.1 are
passed to the first lock-in amplifier 28, the electrical signals at
.omega..sub.2 and 2.omega..sub.2 are passed to the second lock-in
amplifier 30, and the electrical signals at .omega..sub.3 and
2.omega..sub.3 are passed to the third lock-in amplifier 32. The
lock-in amplifiers 28, 30, 32 are thereby set to measure the
amplitudes of the first and second harmonic frequency components of
the photodetector output signal corresponding to the first LPG 12,
the second LPG 14 and the third LPG 16 respectively.
The ratio of the first and second derivatives is a unique function
of position within the spectral transmission profile and is
independent of any attenuation which may occur within the optical
system. The amplitudes of the first and second harmonic frequency
components can be represented by: Amp.sup.1st=A sin(.zeta.) and
Amp.sup.2nd=B sin(.zeta.+.alpha.) where .zeta. represents the
degree of curvature experience by an LPG under interrogation and
.alpha. is the relative phase difference between the first and
second harmonics. The ratio of the amplitudes of the harmonics is
unique for a given radius of curvature, as illustrated in the
parametric plot of the first and second harmonics recorded for
radii of curvature of between 0 and 4.20 m.sup.-1 shown in FIG.
4.
A more useful relationship between the amplitudes of the first and
second harmonic frequency components of the photodetector output
signal and the radius of curvature applied to an LPG is produced by
taking the inverse tangent (arctan) of the ratio of the amplitudes
of the first and second harmonics. This yields an approximately
linear relationship between the arctan of the ratio of amplitudes
and the radius of curvature, as shown in FIGS. 5 and 6 for the
first LPG 12 and the second LPG 14 respectively. A radius of
curvature resolution of +/-0.05 m.sup.-1 and a curvature
measurement range of .about.+/-3 m.sup.-1 is available in this
example.
The spectral sensitivity
d.lamda.d ##EQU00001## of the third LPG 16, i.e. the change in the
central wavelength (.DELTA..lamda.) of the attenuation band
associated with the fourth cladding mode, as a function of radius
of curvature (R) is 3.747.+-.0.002 nm.m, and is a linear
relationship, as shown in FIG. 9. The theoretically predicted
wavelength shift is given by:
.DELTA..lamda..LAMBDA..lamda..delta..times..times..delta..times..times.d.-
delta..times..times.d.delta..times..times..delta..times..times..times.d.LA-
MBDA.d.DELTA..times..times. ##EQU00002## where
.delta.n.sub.eff=n.sub.coeff-n.sub.cleff.sub..nu.is the
differential effective index between the cladding and the core mode
and .delta.n.sub.g=n.sub.cog-n.sub.clg.sub..nu.is the differential
group index. The effective refractive indices of the core and
4.sup.th cladding mode are calculated as a function of curvature
using a 2-D curvilinear hybrid mode eigenvalue equation. It is
assumed that the amount of birefringence induced in a typical
single mode fibre is negligibly small for curvatures of <2
m.sup.-1, so no birefringence induced splitting of the attenuation
band will occur. FIG. 9 demonstrates reasonable agreement between
the theoretical wavelength shift values 56 calculated at a number
of different curvatures and the experimentally measured values
58.
In this embodiment the LPGs 12, 14, 16 are embedded within a
carrier member 40, shown in FIGS. 1, 7 and 8. The carrier member 40
is deformable and takes the form of a skeleton 42 embedded between
two sheets of flexible rubber skin 44, 46. The lower skin 46, which
will be in contact with the surface to be profiled, comprises a
sheet (length 250 mm, width 120 mm and thickness 2 mm) of natural
latex rubber. This provides a flexible stage which is also
thermally insulating. The upper skin 44 comprises a room
temperature vulcanising clear silicon rubber (n>1.5), and has a
thickness of approximately 3 mm.
The skeleton 42 is partially rigid, but expandable, and is
constructed from strips 50, 52, 54 (length 200 mm, width 12 mm, and
thickness 0.254 mm) of carbon steel, which support the sensing
sections of the PTL fibre 18 containing the LPGs 12, 14, 16. The
support strips 50, 52, 54 are fixed to the lower skin 46 and are
arranged parallel to one another, approximately 75 mm apart. Two
connecting strips 56 (length 80 mm) are provided at either end of
the support strips 50, 52, 54. V-section grooves 48 are formed
along the length of the support strips 50, 52, 54 for receiving the
sensing sections of the PTL fibre 18. The fibre is fixed to the
support strips 50, 52, 54 using a cyanoacrylate adhesive. The
v-grooves 48 minimise bending of the LPGs 12, 14, 16 during the
gluing process. The steel skeleton 42 gives longitudinal rigidity
to the carrier member 40 and prevents the LPGs 12, 14, 16 from
experiencing significant axial strain during use.
The steel skeleton also acts to stabilise the temperature of the
LPGs 12, 14, 16. Over a 15.degree. C. temperature range the central
wavelength of the attenuation mode associated with the 4.sup.th
cladding mode of the third LPG 16 was observed to shift by 0.36 nm.
This gives a temperature sensitivity of
d.lamda.d.+-..times..times..times..times..times..times..times..degree.
##EQU00003##
Comparing this with a known temperature sensitivity of 0.198
nm.degree. C..sup.-1 for the same cladding mode in bare PTL fibre,
indicates that the temperature sensitivity of the LPGs 12, 14, 16
mounted on the carbon steel support strips 50, 52, 54 is
approximately an order of magnitude smaller than that of bare PTL
fibre. This reduction in temperature sensitivity of the LPGs 12,
14, 16 is due to the LPGs taking on the thermal expansion
properties of the support strips.
In use, the carrier member 40 is placed on the surface to be
profiled, with the lower skin 46 in contact with the surface. The
radius of curvature of the surface at various monitoring locations
is measured by the LPG provided at the respective monitoring
location. The radius of curvature values measured by each LPG are
input into a surface-modelling algorithm which creates a
2-dimensional or 3-dimensional wire-frame profile of the surface
under investigation. By continually or repeatedly measuring the
radius of curvature at each of the monitoring locations any changes
in the profile of the surface can be monitored. Movement of the
surface at one or more of the monitoring locations can also be
tracked.
Surface profiling apparatus 60 according to a second embodiment of
the invention is shown in FIG. 10. The apparatus 60 is
substantially the same as the surface profiling apparatus 10 of the
first embodiment, with the following modifications. The same
reference numbers are retained for corresponding features.
In this embodiment the LPGs 12, 14, 16 are optically interrogated
by wavelength modulated optical signals generated by illuminating
three fibre Bragg gratings (FBGs) 62, 64, 66 with a broadband
optical source in the form of a superluminescent light emitting
diode (SLED) 68. The optical signal from the SLED 68 is routed to
the FBGs 62, 64, 66 via an optical circulator (or coupler) 76. The
light reflected by each of the FBGs 62, 64, 66 forms a narrow
bandwidth optical signal which is coupled into the PTL fibre 18
through the circulator 76. Each optical signal has a central
wavelength corresponding to the resonant wavelength of the
respective FBG. Each FBG 62, 64, 66 has a different resonant
wavelength, which lies within the spectral profile of its
respective LPG 12, 14, 16. Each FBG 62, 64, 66 is coupled to tuning
means in the form of a piezoelectric based strain apparatus 70, 72,
74 operable to apply an axial strain to the respective FBG 62, 64,
66 at a desired modulation frequency. A wavelength modulation at
that modulation frequency is thereby applied to the resonant
wavelength of the FBG 62, 64, 66.
Similarly to the first embodiment, the modulation signals at
frequencies .omega..sub.1, .omega..sub.2 and .omega..sub.3 are
generated by a signal generator 34. The modulation signals are
applied to the drive voltage supplied from a drive unit 78 to the
piezoelectric element in each strain apparatus 70, 72, 74. The
piezoelectric element in each strain apparatus is thereby caused to
expand and contract at the desired modulation frequency, so
applying a varying axial strain to the respective FBG 62, 64, 66. A
different modulation frequency is applied to each FBG.
FIG. 11 shows surface profiling apparatus 80 according to a third
embodiment of the invention. The apparatus 80 is substantially the
same as the surface profiling apparatus 10 of the first embodiment,
with the following modifications. The same reference numbers are
retained for corresponding features.
In this embodiment the LPGs 12, 14, 16 are optically interrogated
by wavelength modulated optical signals generated by three FBG
fibre lasers 82, 84, 86. The laser cavity of each fibre laser 82,
84, 86 is formed by two FBGs provided in a spaced relationship in a
section of erbium-doped single mode optical fibre; the FBGs and the
fibre forming the laser cavity. The fibre lasers 82, 84, 86 are
pumped by a 980 nm pump laser 88, optically coupled to the
erbium-doped fibre via an optical circulator (or coupler) 90.
Each fibre laser 82, 84, 86 lases at a wavelength determined by the
resonant wavelength of its FBGs. Each pair of FBGs have a different
resonant wavelength to thereby give each fibre laser 82, 84, 86 a
different operating wavelength. The optical output signal from each
fibre laser 82, 84, 86 is coupled to the PTL fibre 18 via the
circulator 90. Each pair of FBGs is coupled to tuning means in the
form of a piezoelectric based strain apparatus 92, 94, 96 operable
to apply an axial strain to the FBGs at a desired modulation
frequency, to thereby apply a wavelength modulation at that
modulation frequency to the resonant wavelength of the FBGs. When
the resonant wavelength of the FBGs in a fibre laser 82, 84, 86
changes, the operating wavelength of the fibre laser also changes.
Therefore, applying an axial strain to the FBGs of a particular
fibre laser 82, 84, 86 at a desired modulation frequency will apply
a wavelength modulation at that frequency to the output wavelength
of the fibre laser 82, 84, 86. A different modulation frequency is
applied to each fibre laser.
As in the first embodiment, the modulation signals at frequencies
.omega..sub.1, .omega..sub.2 and .omega..sub.3 are generated by a
signal generator 34. The modulation signals are applied to the
drive voltage supplied from a drive unit 98 to the piezoelectric
element in each strain apparatus 92, 94, 96. The piezoelectric
element in each strain apparatus is thereby caused to expand and
contract at the desired modulation frequency, so applying a varying
axial strain to the FBGs of the respective fibre laser 82, 84,
86.
Surface profiling apparatus 120 according to a fourth embodiment of
the invention is shown in FIG. 12. The apparatus 120 is similar to
that shown in the first embodiment, but has different optical
interrogation means 122, as described below. The same reference
numbers are retained for corresponding features.
In this embodiment the LPGs 12, 14, 16 are illuminated by a
broadband optical source in the form of a fibre pigtailed SLED 124,
optically coupled to the input end 18a of the PTL fibre 18. The
SLED 124 is operable to generate a broad bandwidth optical signal
whose spectral range encompasses the spectral profiles of the
attenuation bands of each of the LPGs 12, 14, 16. A small, portable
optical spectrum analyser (OSA) 126, such as the "USB2000 Miniature
Fiber Optic Spectrometer" from Oceanoptics Inc., is optically
coupled to the output end 18b of the PTL fibre 18. The OSA 126 is
operable to record the spectral profiles of the attenuation bands
of each of the LPGs 12, 14, 16. The OSA 126 is connected to a
microprocessor 128, which may be a personal computer, to which the
recorded spectral profile data is downloaded. The microprocessor
128 is operable to compare the downloaded spectral profile data
with a plurality of pre-recorded sets of spectral profile data,
until a matching set of spectral profile data is found. Each set of
pre-recorded spectral profile data corresponds to a particular
curvature applied to a particular LPG, so a match indicates the
curvature experienced by the LPG 12, 14, 16 under interrogation.
The OSA 126 may be permanently connected to the microprocessor 128,
so that each recorded set of spectral profile data can be
downloaded to, and processed by, the microprocessor 128 in real
time. Alternatively, the OSA 126 does not have to be connected to
the microprocessor 128 during interrogation of one or more LPGs 12,
14, 16. A number of spectral profiles may be recorded and stored in
the OSA 126 for later downloading to, and processing by, the
microprocessor 128 once the OSA 126 is connected to it.
FIG. 13 shows surface profiling apparatus 130 according to a fifth
embodiment of the invention. The apparatus 130 is substantially the
same as the surface profiling apparatus 10 of the first embodiment,
with the following modifications. The same reference numbers are
retained for corresponding features.
In this embodiment only two LPGs 12, 14 are provided within the PTL
fibre 18. The skilled person will however appreciate that a larger
number of LPGs may be provided. As a result, only two wavelength
modulated optical signals are required to interrogate the LPGs 12,
14. The interrogating optical signals are generated by illuminating
two fibre Bragg gratings (FBGs) 62, 64 with a broadband optical
source, which in this example takes the form of the spontaneous
emission generated by an Erbium doped fibre amplifier (EDFA) 132.
As in the first embodiment, the optical signal from the EDFA 132 is
routed to the FBGs 62, 64 via an optical circulator (or coupler)
76. The light reflected by each of the FBGs 62, 64, 66 forming a
narrow bandwidth optical signal which is coupled into the PTL fibre
18 through the circulator 76. Each FBG 62, 64 is coupled to tuning
means in the form of a piezoelectric based strain apparatus 70, 72
operable to apply an axial strain to the respective FBG 62, 64 at a
desired modulation frequency. A wavelength modulation at that
modulation frequency is thereby applied to the resonant wavelength
of the FBG 62, 64.
In this example, the electrical output of the photodetector 26 is
connected to a single lock-in amplifier 134, operable at multiple
frequencies. The output from the lock-in amplifier 134 is passed to
a data processing and storage device 136.
The curvature sensors, LPGs 12, 14, detect any variations in the
shape of the surface under interrogation. In some instances, when
the surface under interrogation is expandable, for example a human
torso, the positions of the LPGs 12, 14 relative to one another
must also be monitored, in order to take into account variations in
the volume enclosed within the surface. This is achieved here by
providing an optical waveguide strain sensor, in the form of an
fibre Bragg grating (FBG) 138, at a location between the LPGs 12,
14. The FBG 138 measures the strain in the section of PTL fibre 18
between the LPGs 12, 14, from which variations in the distance
separating the two LPGs 12, 14 can be determined.
The skilled person will appreciate that where more than two LPGs
are provided, an FBG strain sensor may be provided between each
pair of LPGs. The FBG 138 or FBGs may alternatively be provided
generally alongside, and substantially parallel to, an LPG 12, 14.
The FBG 138 or FBGs would generally be provided within a separate
optical fibre (not shown), which may be standard single mode
fibre.
In this embodiment the optical interrogation means further
comprises means 140 for interrogating the FBG 138 strain sensor in
reflection. The FBG interrogation means 140 may comprise any of the
know systems for interrogating FBGs, which will be well known to
the person skilled in the art and so will not be described in
detail here. The FBG interrogation means 140 may alternatively take
the form of an optical waveguide grating interrogation system
according to our co-pending European patent application number
02258640.8.
Surface profiling apparatus 150 according to a sixth embodiment of
the invention is shown in FIG. 14. The apparatus 150 according to
this embodiment is substantially the same as the apparatus 130 of
the previous embodiment with the following modifications. The same
reference numerals are retained for corresponding features.
In this embodiment the optical waveguide grating curvature sensing
devices take the form of chirped FBGs 152, 154. Although only two
FBGs 152, 154 are shown, the skilled person will again appreciate
that a larger number of FBG curvature sensors may be provided. The
response, i.e. the change in the spectral profile, of the FBGs 152,
154 to curvature/bending may be further or alternatively enhanced
by fabricating the FBGs 152, 154 such that the amplitude of the
periodic refractive index variation of the Bragg grating is tapered
and/or apodised.
The FBGs 152, 154 are provided within an asymmetric optical fibre
having a radially asymmetric cladding layer, which in this example
takes the form of D-shaped fibre 156.
In this embodiment the FBG strain sensor 138 is provided in a
separate optical waveguide, which takes the form of a standard
single mode optical fibre 158.
FIG. 15 shows surface profiling apparatus 160 according to a
seventh embodiment of the invention. The apparatus 160 is
substantially the same as the apparatus 150 of the previous
embodiment, with the following modifications. The same reference
numerals are retained for corresponding features.
In this embodiment the FBG curvature sensors 152, 154 are
interrogated in reflection. Therefore, instead of the photodetector
26 being located at the distal end of the optical fibre 156, the
photodetector is coupled to a port of the optical circulator 76.
The distal end of the fibre 156 is terminated in an optical dump
162.
Referring to FIGS. 16 to 20, an eighth embodiment of the invention
provides respiratory function monitoring apparatus 100 comprising
surface profiling apparatus 102 which is substantially the same as
the surface profiling apparatus 10, 60, 80, 120 according to one of
the first, second, third or fourth embodiments, with the following
modifications. The same reference numbers as in the first
embodiment are used for corresponding features (the first
embodiment is selected for illustration only, and the skilled
person will understand that the surface profiling apparatus
according to any of the second to seventh embodiments may be used
instead).
As shown in FIG. 16, in this example five LPGs (not shown) are
provided within each of four carrier members 40a d, giving a total
of 20 LPGs. Each set of five LPGs are provided within a different
PTL fibre 104, 106, 108, 110. There are four different optical
arrangements which may be used to deal with this large number of
LPGs. In the first, the attenuation band of each LPG has a
different central wavelength and the LPGs are interrogated by
twenty wavelength modulated, narrow bandwidth optical signals
generated by twenty DBF lasers (as in the first embodiment shown in
FIG. 1), by an SLED and twenty FBGs (as in the second embodiment
shown in FIG. 10), or by twenty fibre lasers (as in the third
embodiment shown in FIG. 11). The wavelength of each optical signal
is different, lying within the spectral bandwidth of the respective
LPG, and each optical signal is wavelength modulated at a different
modulation frequency. The output end of each PTL fibre 104, 106,
108, 110 is coupled to a single photodetector 26, and the
electrical output signal from the photodetector is connected to
twenty lock-in amplifiers, each operating at the first and second
harmonics of the modulation frequency of their respective optical
signals.
In the second optical arrangement the attenuation band of each LPG
again has a different central wavelength. The twenty LPGs are
illuminated by a single broadband optical source, for example an
SLED or an edge-emitting light emitting diode (EELED). The spectral
profiles of the attenuation bands are recorded by a single OSA and
compared to pre-recorded spectral profiles using a microprocessor
(as in the fourth embodiment shown in FIG. 12).
In the third optical arrangement, each LPG within a set of five
LPGs has a different attenuation band central wavelength, the LPGs
in each of the four sets having the same five central wavelengths.
This means that only five narrow bandwidth optical signals, having
five different wavelengths, are required to interrogate all twenty
LPGs, since each optical signal can be used to interrogate four
separate LPGs. By connecting the output end of each PTL fibre 104,
106, 108, 110 to a different photodetector, only five different
wavelength modulation frequencies are required. The photodetectors
thereby identify which carrier member 40a d a signal relates to,
and the modulation frequency identifies the LPG within that carrier
member 40a d, thereby identifying the respective monitoring
location.
The fourth optical arrangement similarly uses four sets of five
LPGs. In this case all of the LPGs can be illuminated using a
single broadband optical source, SLED or EELED. The output end of
each PTL fibre 104, 106, 108, 110 is connected to a different OSA,
since the LPGs within each set are identified by wavelength. The
spectral profiles recorded by the OSAs can be downloaded to a
single microprocessor for processing to determine the curvature
experience by each LPG, as in the fourth embodiment.
In this embodiment the respiratory function monitoring apparatus
100 is intended for use on a human subject and the curvature values
are used to generate a 3-dimensional wire-frame image of the
thoracoabdominal surface. Testing of the apparatus 100 was carried
out on a commercial resuscitation training aid manikin. The manikin
comprises a rigid under-frame over which a polymer skin is
stretched. An inflatable air bag is provided between the frame and
skin, and can be inflated and deflated to simulate expansion and
contraction of the surface of the torso in similar volumetric
proportions to that of breathing.
The number of LPGs required to monitor the respiratory function of
an adult human subject was determined using chest profile data
obtained from a CT imager. The upper chest geometry of a male
subject was reconstructed using a 7.sup.th order polynomial and the
surface area evaluated. This was then compared to the area
estimated using simple trapezoidal and four-point cubic-spline
integrations at lower sampling resolutions in order to obtain an
estimate of the error dependence on the number of monitoring
locations, i.e. the number of LPGs, within the surface profiling
apparatus 102.
FIG. 17 shows a plot (A) of surface area error (calculated using
the trapezoidal integration) as a function of the number of
monitoring locations/LPGs provided across a torso (assuming that
the same resolution is required longitudinally and laterally). The
second plot (B) shows surface area error (calculated using the
cubic-spline interpolation) as a function of number of LPGs.
Considering these values together with generally accepted
volumetric performance standards for spirometry function
respiratory function monitoring devices, confirms that the
respiratory function monitoring apparatus 100 requires in the
region of 20 LPGs (using cubic-spline interpolation) to provide a
similar performance.
The response LPG within one carrier member 40 was investigated,
using an OSA, at various degrees of inflation of the manikin, at
five different locations on the manikin. Each location is
represented by a set of dimensions given in the following table,
and shown in FIG. 18.
TABLE-US-00001 Position on torso d.sub.1 d.sub.2 d.sub.3 Maximum
detected FIG. 18 (X) (mm) (mm) (mm) d.sub.4 (mm) wavelength shift
(nm) 1 90 50 50 250 1.05 2 185 50 50 185 3.18 3 287 50 50 75 0.95
Position on torso p.sub.1 p.sub.2 p.sub.3 Maximum detected FIG. 18
(Y) (mm) (mm) (mm) p.sub.4 (mm) wavelength shift (nm) 4 205 25 165
165 1.98 5 100 128 165 165 1.70
The change in the central wavelength of the LPG's attenuation band
as a function of the peripheral expansion of the manikin's skin
(change in the circumference of the manikin's torso) was also
investigated, shown in FIGS. 19 and 20. The variation in response
of the LPG apparent between locations on the upper and lower chest
regions mimics that which might be expected in a real human
subject, as the expansion of the rib cage has a more significant
contribution at higher levels of ventilation.
FIGS. 19 and 20 show that the spectral response of the LPG as a
function of peripheral expansion of the manikin's skin varies with
location on the torso. The errors shown in these figures correspond
to the spectral accuracy (.+-.0.04 nm) of the OSA used to measure
the change in wavelength and a torso circumference error of .+-.1
cm. The circumference error is an estimate of the variation of the
manikin's skin deformation between each set of results.
As discussed above in connection with the first embodiment, LPGs
are temperature sensitive, although this has been reduced by an
order of magnitude due to the fact that the LPGs 12, 14, 16 are
fixed to steel support strips 50, 52, 54. The temperature
sensitivity of the LPGs will introduce an error into the
measurement process during monitoring of respiratory function,
since the surface profiling apparatus 10, 60, 80 is intended to be
used in close contact with the skin. Assuming a typical skin
temperature variation of .about.32.degree. C. to .about.35.degree.
C. this would generate a maximum wavelength error of .+-.0.035 nm.
Using the maximum detected wavelength shift presented in the above
table, this gives a maximum (worst case) relative error of
.about..+-.3% (position 3) and a minimum relative error of
.about..+-.1% (position 2).
As shown in FIG. 16 the carrier members 40a d are attached to a
garment 114 which is worn by the subject. The garment 114 shown is
illustrative only and would in practice be of a closer fit to the
subject's skin, so that the carrier members 40a d are in close
contact with the skin. Providing the carrier members 40a d on a
garment 114 assists in the correct positioning of the LPGs across
the surface to be profiled, i.e. the torso. The optical
interrogation means 112 in this example is carried by a belt 116
worn around the subject's waist, but it could alternatively be
attached to the garment 114, or be provided with attachment means,
such as a mechanical clip or fleece and hook fastener, by which the
subject may attach the optical interrogation means 112 to an item
of clothing.
The surface profiling apparatus of the described embodiments
provide the advantage of having a curvature spectral sensitivity of
3.747 nm.m and the temperature sensitivity of the LPGs is reduced
by approximately an order of magnitude by mounting them on carbon
steel support strips. The LPGs display negligible axial strain due
to their being fixed to the support strips. The surface profiling
apparatus can be used to distinguish between various geometric
variations associated with different locations on a moving surface,
including a human torso during respiratory movement. The
fabrication of the LPGs in a multi-clad single mode optical fibre,
such as PTL, makes the LPGs insensitive to the refractive index of
a surrounding medium. Each of the described optical interrogation
apparatus is portable, enabling the profile of a surface to be
monitored in a real situation (i.e. outside of a laboratory
environment) and allows the subject on which the surface is located
to move freely during measurement and monitoring.
The surface profiling apparatus does not need to be calibrated for
each surface under investigation. The optical grating sensors
(LPGs) only need to be calibrated once: change in the electrical
output signal as a function of change in curvature experienced by
an LPG.
The respiratory function monitoring apparatus described provides
monitoring apparatus which can assess lung function without the
need for flow measurement at the mouth. The apparatus can also
provide detailed information of the dynamics of chest motion during
breathing. The described apparatus will facilitate further studies
of respiratory physiology, because unlike previously known systems,
it can provide a completely non-invasive and quantitative
appreciation of respiratory function. Using 20 LPG sensors allows
the apparatus to be used to generate a geometrical profile of the
chest and abdomen in three dimensions with the necessary accuracy.
The apparatus provides a curvature resolution of
.+-.2.0.times.10.sup.-2 m.sup.-1 which is a relative error of
.+-.1% over the curvature measurement range of the apparatus.
The respiratory function monitoring apparatus described
re-approaches the less complex, useful two compartment monitoring
technology from a geometrical aspect with a view to enhancing the
performance and adding functionality. The apparatus enables a
3-dimensional profile of the thoracoabdominal surface to be
generated, using an on-body reference. The apparatus enables the
movement of selected anatomical positions on the chest and abdomen
surface to be tracked during breathing manoeuvres, as well as
facilitating measurements of tidal respiratory volume. The
curvature values generated by the apparatus are input into a
surface-modelling algorithm to create a 2- or 3-dimensional
wire-frame image of the thoracoabdominal surface.
The electronic and optical elements of the optical interrogation
means can be made very small and therefore portable. This means
that the respiratory function monitoring apparatus can be attached
to the subject, enabling the subject to move freely without
constraint whilst their breathing is monitored. The apparatus
thereby provides an improved diagnostic tool for continuous
monitoring of patients in a healthcare environment.
Various modifications may be made without departing from the scope
of the invention. Referring to the surface profiling apparatus
itself, a different type of optical fibre may be used to that
described, including multilayer optical fibres having three or more
cladding layers. The structure of the optical fibre may be
asymmetrical about its axis. The LPGs may include sections in which
the grating period is chirped, or may include one or more
phase-shifts within the periodic refractive index modulation, to
provide additional information about the direction of the
curvature. The LPGs may alternatively or additionally be asymmetric
about the axis of the fibre. The LPGs may be replaced by an
alternative optical waveguide grating curvature sensing device,
such as two long period gratings arranged to together define an
in-line Mach-Zehnder interferometer, an optical waveguide Bragg
grating, or two optical waveguide Bragg gratings arranged to
together define a Fabry-Perot etalon.
The coupling means may comprise a different number of carrier
members to that described, and a different number of optical
waveguide grating curvature sensing devices may by provided within
each carrier member. In particular, a single carrier member, of a
size and shape suitable to provide a close fit to the surface to be
profiled, may be used. The arrangement of the LPGs within a carrier
member may be different to that used. The carrier member may have a
different structure to that described, in particular a skeleton may
not be necessary for mechanically strong types of optical fibre,
such as polymer fibre. The skin of the carrier member may comprise
a different flexible material.
In the case of an inanimate subject a carrier member may not be
required, the optical waveguide grating curvature sensing devices
being attached directly to, or embedded within, the surface to be
profiled.
The optical interrogation means may utilise different optical
sources operable to generate a wavelength modulated, narrow
bandwidth optical signal. Also, different optical detection means
may be used to that described. In particular, a different number of
photodetectors may be used and the lock-in amplifiers may be
replaced by a different type of synchronous detector. A different
optical spectrum analyser to that described by be used in
connection with the surface profiling apparatus of the described
fourth embodiment.
Referring in particular to the respiratory function monitoring
apparatus, a different number of carrier members may be used, and
each carrier member may incorporate a different number of LPGs. The
garment incorporating the carrier members may be different to that
described.
* * * * *